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.\" Copyright (c) 2012 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH malloc_usable_size 3 2022-12-15 "Linux man-pages 6.03"
.SH NAME
malloc_usable_size \- obtain size of block of memory allocated from heap
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <malloc.h>
.PP
.BI "size_t malloc_usable_size(void *" ptr );
.fi
.SH DESCRIPTION
The
.BR malloc_usable_size ()
function returns the number of usable bytes in the block pointed to by
.IR ptr ,
a pointer to a block of memory allocated by
.BR malloc (3)
or a related function.
.SH RETURN VALUE
.BR malloc_usable_size ()
returns the number of usable bytes in
the block of allocated memory pointed to by
.IR ptr .
If
.I ptr
is NULL, 0 is returned.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR malloc_usable_size ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH STANDARDS
This function is a GNU extension.
.SH NOTES
The value returned by
.BR malloc_usable_size ()
may be greater than the requested size of the allocation because
of alignment and minimum size constraints.
Although the excess bytes can be overwritten by the application
without ill effects,
this is not good programming practice:
the number of excess bytes in an allocation depends on
the underlying implementation.
.PP
The main use of this function is for debugging and introspection.
.SH SEE ALSO
.BR malloc (3)
